The present disclosure is directed to a method of recovery and the related apparatus for use in recovery of drilling, drill-in and completion fluids used in exploration for hydrocarbons. More specifically, this is concerned with recovery of expensive drilling, drill-in and completion fluids, and especially those which utilize cesium formate as the base fluid. Before discussing that particular fluid, the context of drilling, drill-in and completion fluids use (definitions given below) and recovery should be set forth. Within this document, cesium formate base fluid used in the exploration for hydrocarbons will be often referred to as "drilling fluid".
While it is possible to drill for oil and gas formations using air as drilling fluid, the most common process involves drilling with drilling mud. Drilling mud is routinely a mixture of water and barite. Various and sundry clay constituents may be added to impart added weight to the water, and to provide the desired slickness or wetness. In addition, the drilling fluids used in this process often recover and carry a stream of ground particles. These result from the drilling process and are brought to the surface by the drilling mud. The recovery process typically involves taking heavy particles out of the fluid. This often involves the use of sand removing devices such as shale shakers, desanders and the like. The term "drilling mud" refers to traditional barite muds and the common or traditional muds. The term "drill-in fluids" refers to those fluids which are commonly used while drilling in the projected hydrocarbons production zone. The "completion fluids" are employed in the final stages to remove solids deposited and/or remaining in the wellbore prior to initial production.
A more advanced drilling fluid uses the same ingredients along with oil. Oil is the preferred solvent in many wells, the oil having a value of only about $40 per barrel in 1998 prices. In effect, it resembles diesel oil and has approximately that value.
In some well situations, it is necessary to use synthetic oils. These are sometimes called palm oil. These have the form of lubricants which are more expensive, with prices approaching $300 per barrel. In that instance, a well may have as much as a million dollars worth of drilling fluids circulating in it. Recovery and reclamation of the palm oil (sometimes synthetic in nature) is enhanced by recovery of most of that drilling mud base.
In recent years, a number of conditions have been encountered which require an all together different drilling fluid. The most significantly advanced drilling fluid available today utilizes cesium formate as the base fluid. This fluid (represented in symbolic form CsF) has a value approaching $4,000 to $4,200 per barrel in 1998 prices. It is available in only limited quantity. It provides a drilling fluid which is extremely heavy. Definitions of fluid weight will be given below. In addition to that, it has drilling characteristics which are quite advantageous. Some of these advantages will be given below.
With the great benefit from CsF, and with the great cost, care must be taken to recover and recirculate that kind of drilling fluid. Otherwise, it can not be afforded in wells where its use is normally indicated. Moreover, CsF drilling fluid is manufactured and marketed in a limited quantity because of the difficulties of manufacture and the lack of feed stock for making CsF drilling fluid. Beyond that, the CsF drilling fluid must be treated with great care so that practically all of the CsF is recovered for subsequent use and reuse.
To provide some context to this CsF has an advantage in that it makes a heavier weight drilling fluid. Drilling fluid must have a weight above the base line value defined by water. Water has a weight of about 8 pounds per gallon. When specifying a particular drilling fluid weight, water is the base line or minimum value. The weight of the drilling fluid is extremely important to well control. Well control requires appropriate monitoring of the well and control of the mud weight conditions. Wells are drilled underbalanced to speed up drilling, or overbalanced to prevent fluid leakage into the well. An overbalance situation occurs when the drill bit enters a formation where high pressure is likely to be encountered.
The weight of the fluid in the well borehole is varied depending on the probabilities of drilling into a high pressure formations. The weight of the fluid is varied to take into account these and other factors. Another aspect of the drilling fluid is the manner in which the fluid reacts with the drilling process. When initially used at the start of a well borehole, it is desirable that the drilling fluid segregate into a mud cake by solvent flow to adjacent formations. Explaining, the mud cake is a deposit of the sediment in the drill fluid. In effect, it is the heavy components which form a tacky or adhesive layer against the sidewall. This enhances the isolation of the open hole, thereby restricting the flow of fluids into the adjacent formations. To accomplish this, some portion of the drilling fluid solvent must flow into the formations. It is the filtrate that flows into the formation while the sediment forms the mud cake. This desirable process is important in the initial and intermediate stages of the well borehole. It is especially not desirable when drilling into formations which are likely to start producing flowing hydrocarbons.
Wells are normally deviated from the vertical so that the well borehole entering into the formation penetrates the production formation for greater lengths. Ideally, the borehole is steered so that it follows the producing section of the formation. This takes the borehole through a greater distance in the pay zone. While the vertical hole will have a pay zone only equal to the height of the formation, the pay zone is increased tremendously by drilling along the formation between the top and bottom interfacial boundaries of the producing formation. When drilling in that region, different drilling techniques are also required. It is not enough to merely rotate the drill bit, relying on the weight of the drill string on the drill bit and the stiffness of several drill collars to maintain a true hole with appropriate weight at the same instance. When drilling horizontally, for instance, the weight of the drill string is on the side, and the weight on the bit is substantially nil unless the drill bit is pushed into the well borehole. Moreover, when the hole is deviated towards the horizontal, it then requires that the pipe snake around the corner, so to speak, and that creates great difficulties in applying torque to the pipe string from the rotary table. That is often dealt with by removing the conventional tri-cone drill bit and substituting a mud motor with a specialty drill bit or with a modified tri-cone bit which is somewhat reinforced to accommodate lateral loading. The drill bit is mounted on the mud motor, and well known or conventional steering equipment is used to control the deviation.
In this region, it is desirable to use the premium drilling fluids. In some wells, the quality of the well production is markedly enhanced by using the premium drill fluids. In the selected instances where CsF is used, it has a notable benefit to the production flow rate, and yet continues to maintain the positive well control which is achieved from overbalancing with respect to the formation. With the well fluid maintaining the localized pressure achieved by the column of drilling fluid above the formation sufficient to prevent the formation from flowing into the well borehole, this keeps production fluids from commingling with the drill fluids. If that were to occur, the column of drilling fluid might be reduced in density. This is known by the term "gas cut" which means that small bubbles of natural gas have been commingled in the drilling fluid. As bubbles migrate to the surface, they tend to foam with reduction in dynamic pressure. This can easily create a catastrophic blowout because the aggregate pressure at the bottom become less and less over time, and a run away condition may be encountered in which formation fluids flow freely into the well borehole. This is how blowouts start.
The best way to overcome a blowout is to control the weight of fluid so that the formation faces an overbalanced condition, i.e. bottom hole pressure of the column of fluid in the well borehole is preferably greater than the formation pressure that might be encountered. In deep wells, the formation pressures can be exceedingly high, thereby mandating the use of a heavier column of drilling fluid.
The CsF fluid of the present disclosure has a weight ranging as high as 19 pounds per gallon. That is one of the very heaviest fluids available. It is effective to maintain good control, desirable lubricity in the region of the drill bit, and does not interact in a negative sense with the formation. Such a fluid is relatively expensive, having a cost of about $4,000 per barrel in 1998 prices.
This expensive and highly desirable drilling fluid is used in a well and then must be recovered. After that, it is cleaned up and recycled for use. Present day techniques of cleaning up CsF drilling fluid recover perhaps 66%, and sometimes slightly more of the fluid. In effect, this would mean that a well requiring 1,000 barrels of drilling fluid would involve a capital expense of approximately $4 million for drilling fluid to finish the drilling process and would recover only about two thirds of that value. Even should the recovery be raised to 75%, meaning approximately $3 million worth of drilling fluid, that would still represent a significant cost. The loss is so great that the present disclosure sets forth a method and apparatus for cleaning up the drilling fluid and especially for recovering the expensive CsF.
The CsF recovered by the present process has the form of a water base dissolved salt. It runs the risk of picking up cuttings and other debris during drilling. There will be particles (ranging from the largest to the smallest) which are generated by the drilling process. In addition, there may be salt water from various artesian formations as well as oil, gas bubbles and the like. At the surface, it is necessary to remove all these materials from the heavy salt solution to recover it. By appropriate use of the present method and apparatus, the recovery can be increased significantly so that the percent loss is reduced from about a 30% loss to just a few percent loss depending on adjustments made to the equipment. By use of this procedure, the recovered fluid is clean and pure, and the portions which are discarded include a significantly reduced trace of the CsF in the discarded trash (mostly solids). The segregated trash and other cuttings can be discarded. Moreover, they can be discarded by return to an injection well, or it can be placed in any other waste disposal stream conventional in the oil field. The waste material removed from the CsF is primarily cuttings and includes very little of the CsF.